Symmetries of the QCD

An important feature of the QCD is the conspicuous amount of symmetries of its lagrangian. First and foremost, its lagrangian is invariant under local gauge transformations, i.e. one can redefine the quark fields indipendently at every point in the space-time, without changing the physical content of the theory. This determines a number of implications in … Continue reading Symmetries of the QCD

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Colour confinement

The term confinement indicates that coloured free particles do not exist in Nature. Besides valence quarks, a hadron contains a population of more quarks and gluons. From the (b) side of the Figure in the previous post (*) it is possible to infer that the strong interaction between two quarks enhances with the increasing of their … Continue reading Colour confinement

Couplings and asymptotic freedom

Each of the four fundamental interactions can be characterized by a dimensionless parameter expressed in terms of universal constants, the coupling constant. In the lagrangian of strong interactions, the coupling constant is parameterized by the gauge coupling parameter,, giving in natural units . However, in a general field theory, the effective coupling constant is not a constant, … Continue reading Couplings and asymptotic freedom

Why a standard model? Some fundamentals

Until 1945, particle physics was mostly based on the study of cosmic rays. The turning point came with the advent of accelerators and the subsequent particle proliferation. Elementary (to our knowledge) particles can be classified according to their spin statistics, and thus in order to whether they participate in strong interactions, as shown in the Table: Strongly interactive particles are … Continue reading Why a standard model? Some fundamentals

Gauge? Who is he?

No, gauge does not indicate the name of a famous theorist. The word is to be considered a historical misnomer for phase. Gauge theories are a special class of relativistic quantum field theories based on the idea that some transformations (e.g. phase transformations) leave unchanged the Lagrangian of the system (symmetries).  Gauge invariance (or gauge symmetry) is the property … Continue reading Gauge? Who is he?